There has been a continuous demand for an improved FM discriminator especially for wide band (excess of 2 MHz) frequency modulated signals. This is especially true in the case of FM receivers designed to receive the rather weak television signals from satellites. The discriminator in such a case must have excellent linearity, be low in cost, easily reproducable in quantity, and have maximum sensitivity. Preferably the bandwidth of the discriminator should be subject to change in a simple and effective way.
The prior art teaches that frequency modulated signals may be demodulated by splitting the received signals into two paths, delaying the signals in one of the paths by one-quarter wavelength, and employing a phase detector that compares the phase differences in the signals at the outputs of said paths, respectively, and produces a signal varying in amplitude in accordance with the variations in said phase differences.
The concept of delaying a signal by one-quarter wavelength and comparing its phase with that of a similar signal that was not subject to delay has been commonplace, in the art of FM deiscriminators, for many years.